As with most big systems there are degrees. Most of the code isn't in the fast paths so can be written in whatever language makes sense for the developers. For all the love it receives on HN, Java has plenty of practical success as a development language.
The majority of the fast paths have performance characteristics that Java is suitable for as long as it's actually executing your program. hence the focus on GC, networking and concurrency primitives. And mostly Java is chosen where the bulk of the rest of the codebase is Java.
Where you need ridiculously fast, then you have to drop down to what suits. Be that C, ASM or FPGAs. But these have costs wrt dev time and/or ease of establishing correctness. So they tend to be used judiciously.
The majority of the fast paths have performance characteristics that Java is suitable for as long as it's actually executing your program. hence the focus on GC, networking and concurrency primitives. And mostly Java is chosen where the bulk of the rest of the codebase is Java.
Where you need ridiculously fast, then you have to drop down to what suits. Be that C, ASM or FPGAs. But these have costs wrt dev time and/or ease of establishing correctness. So they tend to be used judiciously.